The operation of axial flow gas turbine engines involves the delivery of compressed air to the combustion section of the engine where fuel is added to the air and ignited, and thereafter delivered to the turbine section of the engine where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor. Accordingly, the efficiency of gas turbine engines is dependent in part on the ability to minimize leakage of compressed air between the turbine blades and the shroud of the engine's turbine section. To minimize the gap between the turbine blade tips and the shroud, turbine blades often undergo a final rotor grind such that the turbine rotor assembly closely matches its shroud diameter. As a result, some degree of rubbing with the shroud typically occurs during operation due to manufacturing tolerances, differing rates of thermal expansion and dynamic effects.
Turbine blades alloys are primarily designed to meet mechanical property requirements such as creep rupture and fatigue strengths. However, many turbine engines must operate under conditions which promote hot corrosion and oxidation of the turbine blades formed from such alloys. Therefore, to enhance their environmental resistance, an aluminide or overlay coating is often applied to the blades in order to provide a protective and adherent layer of alumina scales. However, the above-noted machining and rubbing to which the blades are subjected often results in the removal of the aluminide or overlay coating at the blade tips. As a result, the underlying blade material is exposed, leading to corrosion and/or oxidation that causes tip recession or failure, which potentially leads to performance losses due to higher leakage between the blades and the shroud.
From the above, it can be appreciated that both new and worn turbine blades could benefit from being equipped with blade tips which are alloyed to be inherently resistant to oxidation and hot corrosion, such that removal of the blade's aluminide or overlay coating at the blade tip would not effect the environmental resistance of the blade.
While blade tip repair methods are known in the art, as evidenced by U.S. Pat. No. 4,808,055 to Wertz et al., such methods have not identified blade tip alloys which satisfy the mechanical and environmental properties required for operation in a gas turbine engine, while also being capable of being reliably bonded to the turbine blade.
In particular, prior art blade tip alloys which exhibit desirable mechanical properties, such as high temperature stress rupture life, and environmental properties, such as resistance to hot corrosion and oxidation, have been prone to microcracking during deposition onto the turbine blade. Conversely, other prior art alloys have been identified which exhibit adequate tungsten inert gas (TIG) welding or laser fusing characteristics, but do not have the requisite mechanical and/or environmental properties.
Accordingly, it would be advantageous to provide an improved blade tip alloy for turbine blades of gas turbine engines, in which the blade tip alloy is characterized by suitable mechanical properties such as high temperature stress rupture life and desirable environmental properties such as resistance to oxidation and hot corrosion, while further having desirable weldability characteristics.